1. Field of the Invention
This invention relates to the correction of hyperopia, astigmatism, and irregular optical aberrations by changing the shape of the cornea. Specifically, it relates to laser thermal keratoplasty (LTK), where a laser beam is used to heat selected areas of the cornea and cause local shrinkage.
2. Background of the Related Art
Various methods of changing corneal curvature have been developed. In incisional keratotomy, radial, arcuate, or other patterns of incision are made on the corneal surface. These incisions weaken the structural integrity of the cornea and can result in daily refractive fluctuation and long-term refractive shift. Furthermore, surgical errors can result in corneal penetration and intraocular infection.
In mechanical keratomilieusis procedures, a mechanical means is used to remove corneal tissue in the central optical zone. These methods have poor predictability in both the correction of myopia and hyperopia, and can result in severe surgical complications.
In photorefractive keratectomy (PRK), a laser is used to ablate corneal tissue in the central optical zone. A major shortcoming of PRK is the development of haze in the central optical zone. Another shortcoming of PRK is the relatively poor results in the treatment of hyperopia; regression is severe unless a large area of the cornea is ablated. Unlike methods mentioned above, thermal keratoplasty does not involve the undesirable complications relating to incision or excision of corneal tissue.
In thermal keratoplasty, heat is applied to portions of the corneal stroma to produce collagen shrinkage. Corneal stromal collagen is known to shrink to approximately one third of its original length when heated to a temperature range of 60.degree. C. to 65.degree. C. At higher temperatures, substantial additional shrinkage does not occur, but thermal injury and necrosis may result. Heat can be applied to the cornea with surface probes, penetrating probes, electrical probes, ultrasound, and laser light. Laser heating is the ideal choice because the magnitude, depth and pattern of heating can be more precisely controlled and rapid noncontact application is possible.
Several methods of laser thermal keratoplasty (LTK) have been described. The earliest patent concerning laser LTK, issued to Sand (U.S. Pat. No. 4,976,709) taught the use of optical radiations in the wavelength range of 1.8 to 2.55 microns for the shrinkage of collagen tissues. It also specified the use of laser radiation with corneal-collagen absorption coefficient of 15 to 120 cm.sup.-1 for keratoplasty. It was taught that a pulsed application of about 100 msec duration is preferable. Sand further taught the use of means for measuring corneal shape before and after application of laser energy to determine the desired and resulting alteration in corneal refraction. It described experiments using circular or linear arrays of circular dots. Sand further taught the use of various chemical agents to reduce the threshold shrinkage temperature of tissue. In the Sand series of patents, block diagrams of a laser delivery system were provided, but no specific optical arrangement or method of operation was described.
U.S. Pat. No. 5,263,951 to Spears, et al. taught the use of a laser delivery system that engages the cornea and produces a variety of irradiation patterns for correcting myopia and hyperopia. The patterns described include patterns of central disk, annular rings, radial lines, and round dots. The patent presents data obtained using a Co:MgF.sub.2 laser that produced 0.5 to 2.0 W continuous-wave output. However, in the data presented, much higher powers were used, see, for example, in line 2 of Table C of the patent, 5.3 Diopters of hyperopic correction was produced with an annular pattern with 7 mm outer diameter and 5 mm inner diameter with a fluence level of 1.0 Joules/mm.sup.2 delivered in 1.0 second. This translates to a 38 W laser power. Furthermore, the data described was markedly inefficient compared to results described in Moreira, et al., Holmium Laser Thermokeratoplasty, Ophthalmology, Vol. 100, pp. 752-761, 1993.
In Moreira, et al., a similar 6 mm diameter circular treatment pattern using 32 spots of 410 micron diameter and 9 Joules/cm.sup.2 produced 7 diopters of hyperopic correction. This means in Moreira, et al. only 1% of the energy used in U.S. Pat. No. 5,263,951 was used, yet resulting in greater refractive correction.
U.S. Pat. No. 5,348,551 to Spears describes an apparatus similar to U.S. Pat. No. 5,263,951, with the difference in that the intended effect of irradiation is keratocyte killing rather than collagen shrinkage. The data presented in the patent show highly variable results in a rabbit study.
U.S. Pat. No. 5,334,190 to Seiler taught the use of a contact laser probe for the delivery of focused laser energy onto the cornea to cause collagen shrinkage. This contact probe limits the irradiation pattern to a series of round dots.
U.S. Pat. No. 5,281,211 to Parel, et al. taught the use of a noncontact laser delivery system for LTK that utilized axicon optics to form a pattern of circular laser spots on the cornea.
In general, the laser delivery systems previously described can be divided into two categories, contact and noncontact. Noncontact delivery is easier to apply, faster and more comfortable for the patient. Of the prior art patents, only U.S. Pat. No. 5,281,211 to Parel, et al. provides an optical system for noncontact delivery. In U.S. Pat. No. 5,281,211, the treatment laser beam is projected simultaneously to a pattern of several treatment areas on the cornea. Consequently, a high laser power output is necessary to achieve the desired corneal stromal heating before significant heat diffusion out of the irradiated area occurs. This precludes the use of diode lasers because diode lasers of suitable wavelengths for LTK currently do not possess the required high peak powers. Nevertheless, diode lasers are attractive light sources because of their low cost, compactness and reliability. A cw InGaAsP/InP diode laser that is capable of emitting 0.5 W at 1.8-1.96 micron wavelength is currently available (SDL-6400, SDL Inc., San Jose, Calif.). This range of wavelength has absorption lengths in water in the range of the typical human corneal thickness, which is highly desirable for LTK. However, a noncontact LTK system that can operate with only 0.5 W of laser power has not been previously described.
It is generally acknowledged that the regression of refractive changes is the main drawback of LTK at this time. The desired refractive change of LTK has been reported to undergo large regression over a period of months.
Despite evolutionary improvements in LTK methodology, significant regression still occurs in all dot patterns of LTK reported so far. The currently available commercial LTK systems from Summit, Sunrise and Technomed all use a ring or concentric rings of laser dot heating, which do not optimally distribute the heating in the corneal stroma to reduce stress concentration. Thus an improved pattern of laser application is needed to optimize the stability of LTK results.